Non-contact inline monitoring of thermoplastic CFRP tape quality using air-coupled ultrasound

Abstract

Abstract.Beginning with the aerospace industry, fiber reinforced plastics have spread towards many applications such as automotive, civil engineering as well as sports and leisure articles. Their superior strength and stiffness to mass ratio made them the number one material for achieving high performance. Especially continuous fiber reinforced plastics allow for the construction of structures which are custom tailored to their mechanical loads by adjusting the paths of the fibers to the loading direction. The two main constituents of CFRP are carbon fibers and matrix. Two possibilities for matrix material exist: thermosetting and thermoplastic matrix. While thermosetting matrix may yield better properties with respect to thermal loads, thermoplasticity opens a wide range of applications due to weldability, shapeability, and compatibility to e.g. injection molded thermoplastic materials. Thin (0.1 mm) thermoplastic continuous fiber CFRP tapes with a width of 100 mm were examined using air-coupled ultrasound. Transducers were arranged in reflection as well as transmission setup. By slanted incidence of the ultrasound on the tape surface, guided waves were excited in the material in fiber direction and perpendicular to the fiber direction. Artificial defects -fiber cuts, matrix cuts, circular holes, low velocity impacts from tool drop, and sharp bends -were produced. Experiments on a stationary tape showed good detectability of all artificial defects by guided waves. Also the effects of variation in material properties, fiber volume content and fiber matrix adhesion being the most relevant, on guided wave propagation were examined, to allow for quality assessment. Guided wave measurements were supported by destructive analysis. Also an apparatus containing one endless loop of CFRP tape was constructed and built to simulate inline testing of CFRP tapes, as it would be employed in a CFRP tape production environment or at a CFRP tape processing facility. The influences of tape conveying speed on detectability of artificial defects as well as material properties were elaborated and recommendations for implementation in production scale inline monitoring are given.

A novel measurement approach based on optical coherence tomography for inline quality assessment of thermoplastic glass-fiber reinforced unidirectional tapes

“…Essig and Kreutzbruck successfully measured differences in impregnation quality by air-coupled ultrasonics. 11 This method was also used by Essig and Fey et al 12 on a special inline test rig to detect artificially induced defects (fiber cuts, matrix cuts, circular holes, impact damages, and sharp bends) at tape speeds of up to 5.35 m/min.…”

Air-Coupled Reception of a Slow Ultrasonic A0 Mode Wave Propagating in Thin Plastic Film

“…From Equation (1), it follows that the phase velocity c A0 depends on the fd values. At fd values ≥500 and materials with high Young's modulus values, such as aluminum foil and composite materials, the phase velocity is higher than the ultrasound velocity in air, and the Lamb waves in thin films are excited according to Snell's law [29]. At lower fd values at low frequencies, in the thin plates for some materials, f and/or the A 0 mode phase velocity can become slower than the ultrasound velocity in air.…”